Optic probes for quantitative determination of certain chemical properties of the blood in situ, are known. Thus, in U.S. Pat. No. 4,476,870 (Peterson), which issued Oct. 16, 1984, and the contents of which are incorporated by reference herein, there is described such a probe intended for determining the partial pressure of oxygen in the blood or tissue of a living animal. This probe comprises one or two strands of plastic optical fibers terminating in an elongate section of porous polymer tubing which is packed with a fluorescent visible light-excitable dye placed on a porous adsorptive particulate polymeric support. While many compounds which are excited by ultraviolet light are known, and the intensity of the emitted light of such compounds may be sensitive to the presence of oxygen, Peterson used plastic optical fibers, because (inter alia) he regarded the use of inorganic fibers as impractical for the desired purpose because of brittleness. Also, since plastic optical fibers are insufficiently transparent to ultraviolet light, this necessitated the use by him of visible light and of dyes sensitive thereto.
As will be seen from the description herein, and contrary to the teaching of Peterson, the present inventors have found the use of certain inorganic optical fibers to be eminently practical for the purposes of the present invention, and brittleness is not a problem. Moreover, the use of a sensitized porous glass tip at the end of a glass optical fiber, enables the manufacture of optical probe apparatus which is believed to be considerably more sensitive than that of Peterson. Furthermore, the use according to the present invention of such a tip has the result that the sensor can be much smaller than, and is therefore potentially of greater flexibility and applicability than Peterson's probe. Unpublished experiments by the present inventors have shown that in fact no more than an ambit of about 200 microns of Peterson's probe is the realistic sensitive volume.
In U.S. Pat. No. 4,568,518 (Wolfbeis), which issued Feb. 4, 1986 (the contents of which are incorporated by reference herein). there is described a flexible sensor element comprising a carrier membrane and an immobilized network structure (especially one based on cellulose) including a fluorescent indicator, and in particular such an indicator for measurement of pH values and for blood gas analysis. Wolfbeis's object appears to be to load the carrier with as much indicator as possible and he takes the view that "all known methods of immobilization pertaining to glass surfaces suffer from the disadvantage that the surface will take up only a relatively small amount of bonded immobilized material in a single layer".
Contrary to the teaching of Wolfbeis, however, the present inventors have found that the fact that glass surfaces may only immobilize a relatively small amount of material (in the present invention the relevant material is adsorbed on internal surfaces of the glass, optimally in a single layer) is to be regarded as an advantage and not a disadvantage. This is because when much more than a molecular layer of fluorescent material is immobilized on a carrier, the fluorescent molecules under excitation may tend to react physically with each other, the effect of which will be to substantially reduce the number of excited molecules which give the desired information.
In European Patent Application No. 0214768 (Hirschfeld), published Mar. 18, 1987, the contents of which are incorporated herein by reference, physical and chemical properties of a sample fluid are monitored by measuring an optical signal generated by a fluorescent substance and modulated by an absorber substance. In practice, both fluorescent and absorber substances may be adsorbed on or/and covalently bonded to glass in the form of porous or sintered glass, or a colored filter glass may be used alternatively as substrate, the substrate in each case being attached by adhesive to one end of a fiber optic. There is no suggestion in this European Patent Application either that the fluorescent substance may be used in absence of the absorber substance, or that the device of the invention may be used for the measurement of chemical properties of the blood in situ, although the same inventor in U.S. Pat. No. 4,599,901, which issued July 15, 1986 (and the contents of which are incorporated by reference herein) described a method for direct measurement of arterial blood pressure measuring the intensity of emissions from (inter alia) the surface of a plastic bubble coated with a fluorescent composition and attached to the end of a fiber optic. Moreover, the teaching of the use of porous glass in EP 0214768 is restricted to the use of commercially available material which is attached by adhesive to the fiber optic; any concept of attachment in any other manner, or of formation of the porous glass sensor element in situ is completely absent.
In principle, the present invention provides an optical probe device, of which the essential elements are (except for an optional surface polymeric film) virtually completely fabricated from glass without the aid of adhesive, and which it is believed, when having adsorbed on the internal surface thereof one or more light-sensitive substances, are more sensitive than comparable prior art sensors. The sensor element is also much smaller than hitherto, and this enables the device to be used in situations, especially in relation to monitoring the chemical properties of the human blood stream in vivo, which it is believed were not from a practical standpoint possible before the advent of the present invention.
It will be seen infra that the present invention makes use of substantially non-porous glasses ("parent glasses") which are convertible to porous glasses. Both parent glasses and porous glasses are well known in the art. When parent glasses, which may be certain borosilicate glasses, are heat-treated there results an interconnected separation of phases, one of which may be leached by acid (or in certain cases even by water) to leave an insoluble mainly silica phase (in fact, a porous glass) which could be consolidated by heating into a dense, clear glass known in the trade as "Vycor". Since it is the porous glass which is the desirable carrier for light-sensitive substance in accordance with the present invention, any such consolidation step as is used to produce "Vycor" glass is of course omitted herein. Composition ranges for parent glasses, which are to be regarded as illustrative only, are:
(1) SiO.sub.2 55-70, Na.sub.2 O10-0.1, B.sub.2 O.sub.3 balance to make 100%; PA1 (2) SiO.sub.2 55-70, K.sub.2 O9-0.1, B.sub.2 O.sub.3 balance to make 100%; PA1 (3) Al.sub.2 O.sub.3 0.1-4, SiO.sub.2 [55 minus 1.25.times.Al.sub.2 O.sub.3 content] up to 70, Na.sub.2 O 10-0.1[minus 0.17.times.Al.sub.2 O.sub.3 content], B.sub.2 O.sub.3 balance to make 100%; PA1 (4) SiO.sub.2 55-75, alkalis 5-15, oxides of Fe, Co, Ni 5-15, B.sub.2 O.sub.3 15-30%.
Literature references to porous glasses are also included in the above-cited European Patent Application.
In GB 1190583 (Bergman), published May 6, 1970, there is described a gas detector for measuring or monitoring the partial pressure of a gas (in particular, oxygen), containing a matrix support for luminescent material. The matrix, which in practice is used in the form of a thin film or disc, may be made of "porous Vycor-type glass". No details are provided of how such glass is obtained. In his illustrated embodiment, a sintered metal cylinder contains the matrix, an ultraviolet glow lamp, filters and photoelectric cells. The atmosphere to be monitored either diffuses through the wall of the cylinder, or is led through the apparatus by inlet and outlet pipes. This patent is not concerned with the use of fiber optics, with a detector element of such a size that it may constitute or be attached to the tip of a fiber optic, or with monitoring the chemical properties of the human blood stream in vivo.